Police stand outside of YouTube headquarters in San Bruno, California, after a shooting there last year.

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Some of tech's biggest names, including Airbnb, Pinterest, Twitter and Uber, have signed onto a letter sent to the US Senate from 145 well-known companies demanding action on gun control. But Amazon, Apple, Facebook, Google and Microsoft weren't among them.

"Doing nothing about America's gun violence crisis is simply unacceptable and it is time to stand with the American public on gun safety," the company heads wrote in the letter, which was published on the website CEOs for Gun Safety and originally reported by The New York Times.